Safety plug for pressure vessels



1967 J. J. EIMER 3,356,257

SAFETY PLUG FOR PRESSURE VESSELS Filed Sept. 16, 1965 United States Patent 3,356,257 SAFETY PLUG FOR PRESSURE VESSELS John J. Eimer, Manchester, Mo., assignor to Midwest Consultants, Inc., St. Louis, Mo., a corporation of Missouri Filed Sept. 16, 1965, Ser. No. 487,670 11 Claims. (Cl. 22089) ABSTRACT OF THE DISCLOSURE In an aerosol bomb, a safety plug seated in an opening, the safety plug being made of a thermoplastic material of a character that softens gradually with increasing heat. The plug has a web with an area of predetermined weakness whereby in response to heat and to pressure inside the vessel the web deforms gradually, rupturing slowly at the are-a of weakness, releasin the contents of the vessel gently. The plug with a radially outwardly projecting flange to engage the opening-defining wall, and a skirt depending toward the outside of the bomb. A cup outboard of the skirt, with the holes in it.

Background of the invention This invention relates to pressure vessels. It has particular, though not exclusive, application to pressure vessels of the aerosol bomb type, and will be described as applied to aerosol bombs.

The dangers attendant the exposure of aerosol bombs to high temperatures, as in incinenating them, have long been recognized, as is reflected in the notices attached to aerosol containers for household use, and also in a large number of patents upon devices designed to minimize these dangers.

Many of the prior art patents have to do with fusible plugs, constructed in much the same Way as fusible plugs for boilers, hot Water heaters, and the like. One of the chief difiiculties with the fusible plugs suggested heretofore has been that the plugs tend to give way practically instantaneously at some critical temperature. Another difliculty has been that the plugs have been so designed that when they do release, the opening through which the fluid, under high pressure, escapes, acts as if it Were a rocket nozzle, propelling the bomb with dangerous force.

One of the objects of this invention is to provide a pressure vessel of the aerosol bomb variety which, when heated above a safe temperature, acts to relieve pressure inside the container slowly and safely.

Another object is to provide such a container which is easy to manufacture, fill and use, using existing machinery for its construction and filling.

Other objects will become apparent to those skilled in the art in the light of the following description and accompanying drawing.

Summary of the invention In accordance with this invention generally stated, a pressure vessel is provided with a wall having an aperture defined by a substantially planar boundary and a safety plug is provided, seated within the aperture, the plug being made of a thermoplastic material which is resilient, non-frangible at normal temperatures and which gradually softens with increasing temperature. The plug has a flange bearing against the inside surface of the wall, and a thin, continuous web closing the aperture. The web is provided with areas of predetermined weakness, whereby, in response to heat and to pressure inside the vessel, the web deforms gradually, rupturing slowly at the area of weakness, and releasing the contents of the vessel gently.

In the preferred embodiment, the plug is made of standard low-density polyethylene, and the areas of weakness are defined by incised scorelines intersecting at the center of the web.

Brief description of the drawing In the drawing, FIGURE 1 is a top plan view of one embodiment of plug of this invention;

FIGURE 2 is a sectional view taken along the line 2-2 of FIGURE 1;

FIGURE 3 is a fragmentary view, partly in section, showing the plug of FIGURES 1 and 2 seated in an aperture in an aerosol bomb type of pressure vessel; and

FIGURE 4 is a fragmentary sectional view of another embodiment of pressure vessel and plug of this invention, and showing, in dotted lines, an exploded view of the plug.

Description of the prefererd embodiment Referring now to the drawing, particularly to FIG- URES 1-3, for one illustrative embodiment of pressure vessel of this invention, reference numeral 1 indicates an aerosol bomb with a cylindrical side wall 2 and, in this embodiment, a bottom wall 3 integral with the side wall 2. The bottom Wall 3 has an upwardly inwardly extending conical section 4 contiguous the side wall 2 and a planar section 5 at the inner upper part of the conical section 4. A radially inner edge of the planar section 5 defines a circular aperture 6, in Which a plug 10 is seated.

The plug 10 of this embodiment consists of a radially outwardly projecting flange 11, a central, continuous Web 12, and an annular, depending skirt 15. The flange 11 is charnfered on its upper external surface, to form a flexible lip 16. The lower surface of the lip 16 is planar to form a sealing surface 17 and extends radially outwardly beyond the margin of the skirt 15. At the upper end of the skirt 15, at its juncture with the flange 11, the skirt is provided with an annular groove 18-, the upper margin of which is defined by a part of the sealing surface 17.

In this embodiment, an area of Weakness of the Web 12 is defined by two slits or scorelines 13 and 14, extending diametrically, at right angles to one another entirely across the central web 12.

The dimensions of the aperture 6 and the plug 10 are such that when the plug 10 is seated in the aperture, the margin of the planar section 5 of the bottom wall '3 projects into the groove 18 in a snap fit, and the sealing surface 17 of the flange 11 fits tightly against the planar top surface of the section 5. The skirt 15 projects below the lower surface of the section 5, but above the plane of the bottom edge of the can as defined at the meeting edge of the bottom 3 and side wall 2.

The outer surface of the skirt 15 is tapered inwardly downwardly, to facilitate the insertion of the plug, as can be seen clearly from FIGURE 3.

The flexibility of the lip 16 permits the sealing surface 17 to accommodate itself, under the pressure of the contents of the can, to any minute irregularity in the upper surface of the planar section, insuring a fluid-(gas and liquid)-tight seal between the sealing surface 17 and the section 5.

In this preferred embodiment, the slits 13 and 14 extend halfway, axially, through the web 12.

Merely by way of illustration, and not by way of limitation, a plug of molded standard low-density polyethylene can be provided with a central web .060" thick with scorelines .03" deep. The central web can be approximately one-half of an inch in diameter, and the overall diameter to the edge of the lip can be approximately three-fourths of an inch. Under these circumstances, in an aerosol bomb of 3 to 24 ounce capacity and diameters of two to three inches, when Freon 12 is used as the propellant, the central web will deform and stretch outwardly as the temperature and pressure increase until the elastic limit of the material at the point of scoring is reached, when the plug will open, to release the contents of the can slowly into the atmosphere at a temperature of approximately 138 F. and a pressure of about two hundred pounds per square inch gauge. In numerous tests, no sign of bulging or rupturing of the container has been exhibited, and the contents have been released without any appreciable propellent force.

It is important that the central web of the plug be of suflicient area to permit the contents to be discharged within a short enough time so that pressures do not build up during the discharge process. Within this limitation, the size of the plug can vary widely. Thus, the plug With the web three-fourths of an inch in diameter, of the illustrative example, has been used with equal success in three ounces and twenty ounce aerosol bombs.

Referring now to FIGURE 4 for another illustrative embodiment, a so-called three piece bomb 41 has a cylindrical side wall 42 and a bottom wall 43 joined to the side wall 42 by a rolled or crimped joint 70.

In this embodiment, the bottom wall 43 has a conical section 44, sloping upwardly inwardly from the bottom edge of the side wall 42. It also has a planar section 45. Between the planar section 45 and the conical section 44, is an annular rib 47. In this embodiment, the bottom is drawn, in its central section, to form a cup 48, of which the planar section 45 is a flat rim. The cup 48 has a cylindrical side wall 49 and a bottom 50. Holes 51 extend through the bottom 50. In this embodiment, the cup 48, along with the holes 51, constitute the aperture, within which a plug 60 is seated. The bottom 50 forms a tamperresistant guard.

The plug 60 as a flange 61, chamfered radially outwardly to define a flexible lip 66, and a central, continuous web 62. The plug 60 also has a depending skirt 65. In this embodiment both the inner and outer faces of the skirt 65 are cylindrical. Upper and lower annular sealing ribs 69 extend around and radially outwardly of the outer surface of the skirt 65. In this embodiment, the under surface of the lip 66 is slightly coned, so that the radially outer edge is slightly below the radially inner edge of the lip, before the plug is inserted in the cup 48.

The dimensions of the plug 60 and the cup 48 are such that the sealing ribs 69 engage the inner surface of the side wall 49 of the cup snugly, and the under surface of the lip 66, when the plug is forced into seated position as shown in FIGURE 4, engages the upper surface of the planar section 54 snugly. The outer edge of the lip is cammed into contact with the annular rib 47. Again, as in the embodiment shown in FIGURES 1-3, the flexibility of the lip 66, in response to the pressure within the vessel, permits the lip to adapt itself to any slight irregularities in the upper surface of the section 45, to insure a tight seal. In addition, in this embodiment, the sealing ribs 69 provide additional protection against leakage. The depth of the cup 48, and the height of the skirt 65 are such as to permit the opening of the central web in response to an increase in temperature and pressure, just as in the embodiment shown in FIGURES l3. Scorelines, of which only one line 64 is shown, are identical to those of the plug shown in FIGURES 1-3.

The operation of the plug and vessel shown in FIGURE 4 is exactly the same as that of the plug and vessel shown in FIGURES 1-3.

Numerous variations in the construction of the plug and vessel of this invention, within the scope of the appended claims, will occur to those skilled in the art in the light of the foregoing disclosure. Other plastics, such as polypropylene, polycarbonate, polystyrene, plasticised acrylic, nylon and other thermoplastics can be used for plugs for particular applications. The depth of the slits defining the areas of weakness can also be varied. For example, in a plug with a central web .06" thick, and

Freon 12 as a propellant, if the scorelines are .02" deep, the plug will release at approximately 240 pounds per square inch gauge, and 155 F., while if the scorelines are .0375" deep, the plug will release at approximately 160 pounds per square inch gauge and F. As has been indicated, while a range of ratios of the diameter of the central web to the diameter of the container of about 1-4 to l-6 has been found eminently satisfactory, the ratio can vary, provided that the area is sufficient to insure the release of the contents of the vessel without an untoward pressure build up because of a restriction of the aperture through which the contents are to escape. Combinations of elements of the two embodiments shown, e.g., the provision of an annular rib at the radially outer margin of the planar section of the bottom of the embodiment shown in FIGURES 1-3, can be used. The type of propellent gas is relevant only to the conditions of pressure and heat-pressure relationship which the safety plug must meet. Thus, compressed gas, e.g. carbon dioxide and nitrous oxide, hydrocarbons, e.g., isobutane, butane and propane, and halogenated hydrocarbons different from Freon 12 (difluoro dichlor-o methane) e.g., Freon 11 (trichloro monofluoro methane) and others, may be used. These variations are merely illustrative.

Having thus described the invention, what is claimed and desired to be secured by Letters Patent is:

1. In a pressure vessel of the aerosol bomb type the improvement comprising a vessel wall having an aperture defined by a substantially planar boundary and a safety plug seated within said aperture, said plug being made of a thermoplastic material which is resilient, non-frangible at normal temperatures, and gradually softens with increasing temperature, said plug having a flange bearing against the inside surface of said wall and a thin continuous web closing the aperture, said web being provided with areas of predetermined weakness, whereby, in response to heat and pressure inside the vessel, the web deforms gradually, ruptures slowly at the area of weakness, and releases the contents of the vessel gently.

2. The improvement of claim 1 wherein the areas of predetermined weakness are defined by score lines intersecting substantially at the center of the web span.

3. The improvement of claim 1 wherein the plug is provided with a skirt depending from the margin of the web in a direction toward the outside of the vessel, said skirt defining, with the flange, a sealing means.

4. The improvement of claim 1 wherein the flange is chamfered on its inner surface to provide a thin, flexible outer edge portion.

5. The improvement of claim 1 wherein the thermoplastic material is polyethylene.

6. The improvement of claim 5 wherein the plug is adapted to release the contents of the pressure vessel at a pressure of between one hundred and fifty and two hundred and fifty pounds per square inch at a temperature of 118 F. to F., said slug having a web approximately sixty thousandths of an inch thick with incised score lines between twenty and forty thousandths of an inch deep in its inner surface.

7. The improvement of claim 6 wherein the plug is seated in an aperture in the bottom wall of an aerosol bomb of between about /2 and 32-ounce fluid capacity, and the central web is approximately /2 inch wide.

8. In a pressure vessel of the aerosol bomb type the improvement comprising a vessel wall having an aperture therein defined by a substantially planar boundary and a safety plug seated within said aperture, said plug being made of a thermoplastic material which is resilient, nonfrangible at normal temperatures and which gradually softens with increasing temperature, said plug having an outwardly projecting flange, chamfered at its upper surface to provide a thin flexible outer edge portion and a flat under surface bearing against the inside surface of said wall, a thin continuous Web closing the aperture said web having defined areas of predetermined weakness, and

a skirt depending from the margin of the web in a direction toward the outside of the vessel.

9. In a pressure vessel of the aerosol bomb type the improvement comprising a vessel bottom Wall having an aperture therein defined by a substantially planar boundary; a safety plug seated within said aperture, said plug being made of a thermoplastic material which is resilient, non-frangible at normal temperature and which gradually softens with increasing temperature, said plug having an outwardly projecting flange, chamfered at its upper surface to provide a thin flexible outer edge portion and a flat under surface bearing against the inside surface of said wall, a thin continuous Web closing the aperture, said web having defined areas of predetermined weakness and a skirt depending from the margin of the web in a direction toward the outside of the vessel, and an upwardly opening cup mounted in the bottom wall of the pressure vessel, said cup having a flat rim which constitutes the planar boundary of the aperture, and a perforate bottom wall.

10. The pressure vessel of claim 9 wherein the cup has a cylindrical side wall, the plug skirt has a cylindrical outer surface and annular sealing ribs, projecting radially outwardly from the said outer surface, engage the inner surface of the cylindrical side wall.

11. In a pressure vessel of the aerosol bomb type the improvement comprising a vessel wall having an aperture defined by a substantially planar boundary and a safety plug seated within said aperture, said plug being made I a thermoplastic material which is resilient, non-frangib at normal temperatures, and gradually softens with i creasing temperatures, said plug having a flange bearir against the inside surface of said wall and a thin COIltllI ous web closing the aperture, said web being provided Wil areas of predetermined weakness, whereby, in response 1 heat and pressure inside the vessel, the web deforn gradually, ruptures slowly at the area of weakness, an releases the contents of the vessel gently, said plug beir adapted to release the contents of said vessel at a pressui of between one hundred and three hundred pounds pt square inch at a temperature of between 101 F. an 215 F.

References Cited UNITED STATES PATENTS 670,794 3/ 1901 Lott. 2,251,345 8/ 1941 Triplett 2208 2,757,843 8/ 1956 Smith. 2,951,614 9/1960 Greene 220-8 3,005,573 10/ 1961 Dawson et al. 2208 3,029,987 4/ 1962 Gronemeyer 220-8 3,145,884 8/1964 Everett 222-89 RAPHAEL H. SCHWARTZ, Primary Examiner. 

1. IN A PRESSURE VESSEL OF THE AEROSOL BOMB TYPE THE IMPROVEMENT COMPRISING A VESSEL WALL HAVING AN APERTURE DEFINED BY A SUBSTANTIALLY PLANAR BONDARY AND A SAFETY PLUG SEATED WITHIN SAID APERTURE, SAID PLUG BEING MADE OF A THERMOPLASTIC MATERIAL WHICH IS RESILIENT, NON-FRANGIBLE AT NORMAL TEMPERATURES, AND GRADUALLY SOFTENS WITH INCREASING TEMPERATURES, SAID PLUG HAVING A FLANGE BEARING AGAINST THE INSIDE SURFACE OF SAID WALL AND A THIN CONTINUOUS WEB CLOSING THE APERTURE, SAID WEB BEING PROVIDED WITH AREAS OF PREDETERMINED WEAKNESS, WHEREBY, IN RESPONSE TO HEAT AND PRESSURE INSIDE THE VESSEL, THE WEB DEFORMS GRADUALLY, RUPTURES SLOWLY AT THE AREA OF WEAKNESS, AND RELEASES THE CONTENTS OF THE VESSEL GENTLY. 